Well I have 2 guesses about how it can be done. Never tested either so I don't know for sure.
1. Use a bunch of conditionals to check which forum id it is and show the image for that forum ID
ex.
<if condition="$forum['forumid']==2">
//show image for forum with the id 2
</if>
With an if statement for each main category forum.
2. Use a naming convention on the images so you can use the forumId as a replacement variable in the image name.
For example name all your images with the naming convention of 'forumhead_2.png' where 2 is the forum ID that image belongs to. Then in the template try something like this.
<img src='forumhead_$forum[forumid].png' />
--------------- Added 1216611484 at 1216611484 ---------------
BTW you would want to add another row to the part of the table in the template 'forumhome_forumbit_level1_nopost' to achieve this as well.